In recent years, wireless communication systems have used a plurality of antennas for transmission and reception of signals in order to increase communication capacity and to improve communication quality. Such transmission and reception scheme using a plurality of antennas is called MIMO (Multi-Input Multi-Output). In particular, a scheme that a reception terminal feedbacks information related to CSI (Channel State Information: propagation path information) to a transmission terminal is called Closed-Loop MIMO or feedback MIMO, which further improves communication characteristics of MIMO.
The reception terminal can measure CSIk for a k-th subcarrier (channel) as shown in Formula 1 based on a relationship between a specific reference signal (xi) transmitted from the transmission terminal at predetermined intervals and a reception signal (yj, i) of the reception terminal. In Formula 1, TxAnt and RxAnt respectively represent the number of antennas of the transmission terminal and the number of antennas of the reception terminal, whereas CSIk represents complex matrix having a dimension of RxAnt×TxAnt. In many cases, the reference signals are inserted in different subcarriers for each transmission antenna in fact, such that the reception terminal can separate reception signals. However, for a simple description, it is assumed here that reception signals and reference signals of all subcarriers are obtained separately by respective antennas.
                              CSI          k                =                  [                                          ⁢                                                                                          y                                          0                      ,                      0                                                                            x                    0                                                                                                                    y                                          0                      ,                      1                                                                            x                    1                                                                              …                                                                                  y                                          0                      ,                                              TxAnt                        -                        1                                                                                                  x                                          TxAnt                      -                      1                                                                                                                                                                y                                          1                      ,                      0                                                                            x                    0                                                                                                                    y                                          1                      ,                      1                                                                            x                    1                                                                                                                                                                                                  y                                          1                      ,                                              TxAnt                        -                        1                                                                                                  x                                          TxAnt                      -                      1                                                                                                                          ⋮                                                                                                                          ⋱                                            ⋮                                                                                                          y                                                                  RxAnt                        -                        1                                            ,                      0                                                                            x                    0                                                                                                                    y                                                                  RxAnt                        -                        1                                            ,                      1                                                                            x                    1                                                                              …                                                                                  y                                                                  RxAnt                        -                        1                                            ,                                              TxAnt                        -                        1                                                                                                  x                                          TxAnt                      -                      1                                                                                                    ⁢                                          ]                                    [                  Formula          ⁢                                          ⁢          1                ]            
As for feedback MIMO, the communication characteristic of MIMO is more improved, as the information of the CSI fed back from the reception terminal to the transmission terminal is more detailed. However, an amount of communication data is more increased as the information of the CSI fed back from the reception terminal is more detailed, resulting in tightening the wireless communication capacity of the system. In order to address such a problem, it has been performed that the transmission terminal and the reception terminal commonly have information of transmission weight and the reception terminal feedbacks only index information (identification information) of the transmission weight corresponding to the CSI to the transmission terminal (that is, the reception terminal notifies the transmission terminal of an index number of transmission weight to be used only), which significantly reduces feedback information. In addition, applying a single transmission weight to a plurality of subcarriers collectively can reduce an index itself of the transmission weight to be fed back, enabling further reduction in the feedback information.
For example, as for UMB (Ultra Mobile Broadband, see Non-Patent Document 1, for example) and E-UTRA (LTE) (Evolved UMTS Terrestrial Radio Access, Long Term Evolution, see Non-Patent Document 2, for example), which are of 3.9 generation mobile communication systems (hereinafter, referred to as “3.9G”), the information of the transmission weight is shared as PM (Precoding Matrix) by the transmission terminal and the reception terminal. A plurality of PMs is defined correspondingly to the number of antennas and the like. The reception terminal selects a suitable PM according to the CSI and provides the transmission terminal with PMI (Precoding Matrix Index), which is an identification number of the PM, as feedback. When receiving PMI from the reception terminal, the transmission terminal controls the transmission weight of the plurality of antennas by using the PM identified by the PMI.
In UMB, for example, a frequency band used for communications is divided into eight subbands, and each of the subbands is divided into eight tiles, each of which is divided into sixteen subcarriers, as shown in FIG. 5. In order to select PM commonly applicable to the plurality of subcarriers, the reception terminal calculates an average value of the CSI (CSIAve) in the subband and the tile as a unit by using Formula 2. Here, NCSI represents the number of subcarriers in the subband, and is 128(8×16) in the subband as the unit and 16 in the tile as the unit. When obtaining the average value of the CSI, the reception terminal selects a PM optimum to the average value of the CSI and provides the transmission terminal with the PMI corresponding to the PM as feedback.
                                                                        CSI                Ave                            =                                                1                                      N                    CSI                                                  ⁢                                                      ∑                                          i                      =                      0                                                                                      N                        CSI                                            -                      1                                                        ⁢                                      CSI                    i                                                                                                                          =                                                1                                      N                    CSI                                                  ⁢                                  (                                                                                    ∑                                                  i                          =                          0                                                                                                      N                            CSI                                                    -                          1                                                                    ⁢                                              Re                        ⁡                                                  (                                                      CSI                            i                                                    )                                                                                      +                                          j                      ⁢                                                                        ∑                                                      i                            =                            0                                                                                                              N                              CSI                                                        -                            1                                                                          ⁢                                                  Im                          ⁡                                                      (                                                          CSI                              i                                                        )                                                                                                                                )                                                                                        [                  Formula          ⁢                                          ⁢          2                ]            
FIG. 6 shows changes of the frequency usage efficiency [bps/Hz] of the feedback MIMO when averaging of the CSI necessary for selection of the PMI is performed in the subband and in the tile as the unit and also when there is no control of the transmission weight by selection of the PMI. As shown in FIG. 6, under the same SNR (Signal to Noise Ratio) of the transmission signal, the communication characteristic is improved by control of the transmission weight. It is also shown that the communication characteristic is further improved when PMI is selected in a smaller unit (that is, not in the subband but in the tile, as the unit).